The present invention relates generally to vehicle accessories affixed to vehicular windshields, and in particular, to an improved method for mounting a rearview mirror assembly to an automotive windshield and the rearview mirror/windshield arrangement produced by that method.
The front glass windshield in cars used in the United States, and in many cars used elsewhere in the world, is an assembly consisting of two glass panels laminated together. Conventionally, the process to form the windshield involves cutting two flat glass panels in the shape desired for the windshield and then bending these two flat glass panels as a matched pair to give them a matched compound curvature. A sheet of plasticized polyvinylbutyral (PVB) polymeric interlayer is then placed between the bent glass panels and the assembly so formed passes into an autoclave where the windshield lamination occurs. Such an autoclave process typically involves a cycle such as:
20 minutes at 180.degree. F.; PA1 20 minutes at 285.degree. F. and 200 psi; and PA1 cool to room temperature.
However, temperatures in excess of about 325.degree. F. must be avoided so as to reduce any deterioration of the interlayer material. The practice of this windshield manufacturing process has fostered the development of methods which allow simultaneous attachment of the supports for interior rearview mirrors in cars.
Following the bending of the flat glass panels, a button, usually composed of sintered steel or diecast zinc, is attached by an adhesive onto the concave surface of the first of the glass panels intended to be directed to the interior cabin of the vehicle. Conventionally, a plasticized PVB film, which is an elastomeric, thermoplastic material and which is a similar material to that used as the interlayer for the formation of the windshield, is used as the adhesive means. At this stage of the process, the attachment of the button is temporary. This temporary attachment is typically achieved by attaching under modest pressure and heat so that the button is securely held for the assembly to proceed to the autoclave process. It is during the autoclave process of the windshield, at which time lamination of the PVB interlayer between the first and the second bent glass panels occurs, that the permanent attachment of the button to the windshield occurs.
PVB film has been extensively used as the adhesive means to mount mirror buttons to windshields, primarily because it is compatible with the autoclaving cycle for windshields. This compatibility allows the windshield manufacturer to economically supply a windshield to the car manufacturer with the mirror mounting button preattached in its predetermined position on the windshield. At the car assembly plant, a rearview mirror 1 is attached to the button such as shown in FIG. 1.
While the above process has obvious commercial advantages, the use of a PVB film as the adhesive means does have some disadvantages. The primary disadvantage of using PVB film as the adhesive for the button is that it is an elastomeric, thermoplastic material with relatively poor load bearing properties. This deficiency was generally not a problem when the assembly weights for interior rearview mirrors were traditionally from about 100 grams to about 200 grams. Today, however, assembly weights of 400 grams or more are common for interior mirror assemblies that incorporate reading lamps, electrochromic cells and circuitry, twilight sentinels, and the like. These new, heavier mirrors fall off or otherwise detach, even during normal use, with a frequency that is commercially undesirable when attached to windshields via buttons adhered to the windshield using a polyvinylbutyral film. This tendency to fall off has limited the use of the windshield mounting process and has contributed to an alternative, more expensive, mounting technique where the rearview mirrors are mounted in the header area above the windshield. This fall-off of windshield-mounted mirrors is particularly a problem in hot climates, such as found in Arizona, where temperatures in the 70.degree. C.-110.degree. C. range are commonly reached by windshield mounting button arrangements. At these elevated temperatures, PVB film softens considerably. This softening exacerbates the inability of PVB films to support rearview mirrors of increased weight. Also, the overall vibration performance of the mirror assembly attached to the windshield via a PVB film adhesive degrades at elevated temperatures.
Presently, there are no known methods available for forming an effective long-term bond between a mirror assembly and a windshield that ensure the adhesion of the mirror assembly to the windshield over many years and through extreme climatic conditions, even under heavy load, that ensure good overall vibration performance at elevated temperatures and that are compatible with the commercial manufacturing process for laminated windshields. The current PVB film adhesive/mirror mounting button arrangement has been proven not sufficiently adhesive to the windshield to render it effective for most types of interior mirror assemblies. Further, the current PVB film adhesive/mirror mounting button arrangement has been proven to exhibit inferior mirror assembly vibration performance, particularly at elevated temperatures.